But it is still unclear if American Marines will join a multinational peacekeeping team now assembling on the ground.
In Washington, the senior U.S. official said on condition of anonymity that there is "certainly the possibility, maybe even a probability" that Marines will be sent ashore.
Defense Secretary Donald Rumsfeld said he and President Bush, vacationing in Texas, discussed Liberia on a secure video link Tuesday, but had no decisions to announce.
The secretary stressed that a leading role was unlikely for American troops.
"(Mr. Bush) has from the beginning said that any role for the U.S. would be to assist (African troops), not to replace them," Rumsfeld said.
On Monday some 200 Nigerian soldiers armed with machine guns and assault rifles arrived at the airport 30 miles outside Monrovia as the vanguard of a 3,250-man intervention force promised by West African nations.
Mr. Bush's spokesman, Scott McClellan, said U.S. officials were "very encouraged" by Monday's deployment.
He pledged U.S. financial and logistical assistance and repeated Mr. Bush's demand that President Charles Taylor step down.
For his part, Taylor reportedly assured Nigerian officials that he would leave Liberia as pledged soon after he cedes power on Aug. 11.
"He even said the place would no longer be safe for him then," said Nigerian diplomat Folu Ogunbanwo, who was present at a private meeting Monday in Monrovia between the Liberian leader and Nigeria Foreign Minister Oluyemi Adeniji.
The Nigerian troops who arrived Monday were greeted by overjoyed crowds who surged onto the rain-slicked tarmac at the airport outside Monrovia and hoisted one Nigerian army officer to their shoulders.
"I think the war is over," said Fayiah Morris, who was in the throng swarming around Nigerian soldiers in camouflage and flak vests as whirring helicopters touched down.
But the troops' arrival Monday did not end the fighting — the sound of gunfire echoed from Liberia's ruined capital. For much of the day, Liberian rebels and Taylor's troops battled across the Old Bridge, separating the capital's rebel-held island port and the government's downtown stronghold.
At one point, rebels taunted their foes, dancing with brooms, doing back flips and waving at Taylor's men. The government troops fired a .50-caliber machine gun mounted on a pickup truck in reply.
Taylor's troops accused rebels of looting before peacekeeping force move in, but arguments over goods among Taylor's AK-47-armed fighters suggested they were doing the same.
The first peacekeepers concentrated on setting up defenses at the airport. Troops won't move into Monrovia until sufficient numbers arrive, the force's Nigerian commander, Brig. Gen. Festus Okonkwo, told reporters.
West African peacekeeping troops deployed repeatedly in Liberia in the 1990s, at times coming under attack from forces led by Taylor, then a rebel leader. According to The New York Times, that force was accused of looting.
Nigerian officers at the airport said they will operate under rules of engagement authorizing them to shoot to protect civilians or themselves.
"If we want to keep peace and we cannot keep peace, it will amount to enforcing peace," Okonkwo said. "Then we'll get back to the people that sent us. They will give us the mandate."
The West African deployment was approved last week by the U.N. Security Council, which also approved a U.S.-proposed resolution to speed a broader U.N. peacekeeping force within months.
The United Nations plans to send "a fairly sizable force" to Liberia, ideally starting on Oct. 1, to replace the Nigerian-led multinational force, Assistant Secretary-General for Peacekeeping Hedi Annabi said Monday.
He said the Liberia mission will likely be modeled on the U.N. peacekeeping mission in neighboring Sierra Leone, which had 17,500 troops at its height and helped end a war that had ravaged the country since 1991.
Taylor has promised repeatedly to yield power since June 4, when a joint United Nations and Sierra Leone court revealed the war crimes indictment against him for supporting rebels in that nation.
Still, Taylor's government has hedged on his promises to go into exile in Nigeria, saying he would leave only when enough peacekeepers are on the ground and when the war crimes indictment against him is dropped.
He is blamed in nearly 14 years of conflict in Liberia that have killed more than 100,000 people, and accused of gun- and diamond-trafficking and other dealings that have fueled conflicts in West Africa.
Mr. Bush, saying he is worried about overstretching American forces already committed to Iraq and Afghanistan, has said no U.S. troops will enter until Taylor leaves.
At a news conference last week, Mr. Bush also said a cease-fire had to be in place before U.S. became involved. And he stressed that in any engagement in Liberia, "the troop strength will be limited, and the time frame will be limited."
The Iwo Jima, an amphibious assault ship, and the Carter Hall, a dock landing ship are the two U.S. vessels that have arrived. The Nashville, a support ship, is en route.